1. Apparatus has been devised to record the principal parameters of the flight performance of tethered fruit-flies in a wind tunnel.
2. Typically these flies achieve level flight (lift = weight) at 200 cm./sec. and a body angle of + 10°. Lift varies directly with body angle except at very high angles; the stroke parameters are invariant with body angle.
3. Evidence is presented suggesting that these measurements are applicable to free flight.
4. The adaptive significance of the absence of a ‘lift-control reaction’ in fruit-flies is discussed.